Coaxial microwave hybrid structures



May 3, 1960 B. M. DwoRK comm. MIcRowAvE HYBRID STRUCTURES Filed May 15, 1956 FIG W2?. mank INVENTOR.

Bevnard M. DwarK Y L@ 4., m

'lll/1111111 United States Pao F COAXIAL MICROWAVE HYBRID STRUCTURES Bernard M. Dwork, Bedford, Mass., assigner to Merrimac Research and Development Corporation, Flushing, N.Y., a corporation of New York Application May 15, 1956, Serial No. '584,984 3 Claims. (Cl. ass-11) In the operation of microwave systems, one of the most useful devices is termed a hybrid tee, comprising four accessible terminals or openings or arms so connected together that power sent into any one of these openings or arms will split equally into two of the others, but itV will not couple at all with the fourth arm. Such structures are conventional with rectangular waveguides. With such waveguides, it is the practice to make two ofv the connected arms non-coupling by reason of the symmetry of the device, and hence without reference to the frequency of the energy to be coupled.

It is an object of this invention to provide a hybrid tee for use in coaxial lines. Its principal feature is that the isolation between the two conjugate arms depends Von symmetry alone, and is, therefore, independent f frequency. This invention, therefore, comprises the structures hereinafter described, embodiments of which are shown in the accompanying drawings.

Fig. 1 is a hybrid tee for use in waveguides which is here illustrated for purposes of analogy, but comprises no part of this invention; Fig. 2 is a cross-sectional view of a magic tee for coaxial lines embodying this invention, the section being taken on the plane of the center line of all the arms; Fig. 3 shows the relationship of the conductors in a section along the line A--A' of Fig. 2, all parts being omitted Vsave the conductors intersected by the section line; Fig. 4 is a similar view of the conductors intersected by the section line B--B of Fig. 2; Fig. 5 is a view similar to Fig. 3, but with a different excitation; Fig. 6 is a View similar to Fig. 2 of a different embodiment; and Fig. 7 is a fragmentary view of a portion of the structure of Fig. 2 showing a modification thereof.

Referring now to Fig. 1, which is a hybrid tee, it is seen that the rectangular waveguides 1 and 2 are collinear. Arm 3 it is a rectangular wave-guide of similar dimensions having its narrow side ends connected to the narrow side of waveguides 1 and 2, and having its broad faces connected to the broad faces of the conductors 1 and 2. The waveguide 4, similar in cross section to the guide 3, is inserted transversely in the broad face of the guides 1 and 2 at the center of the guide 3. With this construction, the guide 3 connects with the guides 1 and 2 through a long longitudinal opening in the narrow side thereof, and the guide 4 communicates with the guides 1 and 2 through a short opening in the broad side thereof. With such a construction, and appropriate symmetrical matching adjustments, energy directed into any one of the four openings will split between two of the other three and will have no effect of excitation upon the fourth.

A need exists for a practical construction of this type in coaxial lines, and it is an object of this invention to provide such a coaxial structure.

In Fig. 2, four coaxial lines or arms are joined together yas follows: Arms 1 and 2 are collinear and are comprised of an inner conductor 11 which extends thru both the f* 2,935,702 'Patented May-3, .1960

and an outer conductor 15 and the arm 4 consists of an inner conductor 16 and an outer conductor 17.

The conductor 14 of arm 3 is connected by means of a strip 18 with the conductor 11, and the outer conductor 15 is connected to the outer conductor 13 by means of a strip 19 and the outer conductor -12 by a strip 20. These three strips, being parallel, constitute in themselves a three-conductor transmission line.

The outer conductor 17 is connected with the outer conductor` 13 by means of a strip 21, and the inner conductor 16 is connected with the outer conductor 12 by a strip 22. The strips 21 and 22 are parallel and constitute a two-conductor transmission line.

Referring now to Figs. 2 and 3, let us consider a wave entering from arm 3, as it reaches the transmission line comprising the parallel strips 18, 19, and 20.

Because of the manner in which the strips are connected to line 3, the mode of transmission at the threeconductor transmission line will be as indicated in Fig. 3; that is, it will be an excitation between the center strip symmetrically outward to both the outer strips. Since,

therefore, strip 18 is connected to thel central conductor 11 and strips 19 and 20 are connected to the outer conductors =13 and 12', the excitation from arm 3 will be divided between the arms 1 and 2 in equal parts. No energy, however, will pass out thru arm 4, since by virtue of symmetry there is lno electlic field distribution across the region between strips 21and 22. If, on the other hand, a `wave is incident from arm 4 only, the distribution will be that shown in Fig. 4, directly exciting arms 1 and 2 in equal amounts but in opposite phase. Howouter conductor 12 of arm 1 and the outer conductor V13 7 of arm 2. The arm 3 comprises an inner conductor 14 ever, the effect of the excitation upon arm 3 will be reected out, since the excitation upon the strip 19 will be opposite that upon the strip' 20.

This same excitation from arm 4 upon the three-conductor line 18, 19, 20 is shown in Fig. 5, which shows that the center strip 18, directly connected to the center conductor 14, is ignored by this eld distribution. Consequently, the wave is reflected by the junction between the three-strip region and the coaxial line of arm 3, and in effect there is formed a length of short-circuited stub line connected across from line 4. It is seen, therefore, that the arms 3 and 4 are isolated from each other and are conjugate in a manner dependent upon symmetry.

If the characteristic impedance of each collinear arm is Z0, an impedance of approximately Zo/Z is presented to the shunt arm and an impedance of approximately 2Z0 to the series arm when matched loads are placed on the collinear arms. These values will be modified by the presence of junction discontinuity effects, but they are reasonable approximations. One form of matching these lines is by means of quarter wave length geometric mean transmission line transformers. Accordingly, the dimensions of the two-strip line are chosen so ,as to yield a characteristic impedance of \/2Z0, and the line is made a quarter wave length long corresponding to the middle of the frequency range of operation. Similarly, the dimensions of the three-strip line are chosen to yield the value ZUM/, and its length to be a quarter Wave length.

The length of the three-strip line might already have been specified in order to optimize the coupling of the arm 4 to the collinear arms. In that case, the additional length required to form a quarter wave length of the proper characteristic impedance may be formed by inserting an uncompensated dielectric bead of appropriate length and position in the coaxial line portion of arm 3.

These requirements with respect to characteristic impedance and quarter wave length lines influence the match properties but in no way affect the isolation between arms 3 and 4.

It may be noted that two chokes 23 and 24 are placed on the collinear arrns l'1 and 2 in Fig. 2. The reason for the chokes is that the strip 22 of the two-strip line connected to arm 4 connects the inner conductorV 16 of arm 4 to the outer conductor 12 of arm 1. Since the outer conductor' 12 of arm 1 is thus excited ofi ground potential, it should be appropriately choked.

The proper operation of this hybrid tee is made possible by the use of the twoand three-strip lines. A hybrid tee requires the combination of a shunt tee (or junction) and a series tee. An ordinary coaxial tee is only a shunt tee; simple series tees are not possible. A series tee (or junction) was fabricated in the abovedescribed devicer by employing the section of two-strip line. However, this junction would have been too weakly coupled to the collinear arms for reasonable operation without the presence of the three-strip section on arm 3. These two-,and three-strip sections serve the additional function of acting as matching transformer sections. In this manner a highly compact device is obtained.

A somewhat different embodiment of the invention is disclosed in Fig. 6, comprising two collinear' coaxial lines, 1a and 2a, which are intersected at the same point by two other coaxial arms 3a and 4a. The arm 1a comprises an inner conductor 31 and an outer conductor 32, and the arm 2a comprises an inner conductor 33 and an outer conductor 34. The arm 3a comprises an inner conductor 36 and an outer` conductor 37, and the arm 4a comprises an inner conductor 38 and an outer conductor 39. The inner conductor 36 of arm 3a is joined at 40 to two parallel strips 41 and 42 which constitute a twostrip line. Strip 41 is connected to the inner conductor 33 and strip 42 is connected to the inner conductor 31. This two-strip transmission line extends into the outer conductor 37 a length equal to one quarter wave length and the outer conductor 37 joins with the outer conductors 32 and 34. The strips 41 and 42 extend beyond the inner conductors 31 and 33 and are joined respectively to the outer conductor 39 and the inner conductor 38 of transmission line 4a. The outer conductor 32 is directly connected to the outer conductor 39 by a strip 43 and a concentric sleeve 44 extends outwardly from the conductors 34 and 32 around the transmission line 4a for a length of one quarter wave length. This sleeve 44 serves as a choke and prevents the inner conductor 36 of arm 3a from being short-circuited to the outer conductor 34 of arm 2a. Moreover, this assists in causing the energy from arm 4a more efciently to excite the junction.

With this construction, it will be clear that energy entering at 3a will excite arms 1a and 2a equally and in phase, but will transmit no energy whatever to arm 4a.

Energy entering at 4a, likewise, will be divided equally between 1a and 2a but in reverse phase, and will transmit no energy to arm 3a.

In the embodiment shown in Fig. 2 at certain frequencies there may be set up a certain excitation between the strip 22 and the conductor 17 below it, resulting in an unwanted dissipation and Vreilection of energy. This may be avoided by inserting a strip 22a, parallel to the strip 22, connecting conductors 17 and 13, and making th length of the twin conductor line, comprising strips y22 and 22a, equal to 1A wave length, thereby producing achoke.

It is desirable also to insert a similar stn'p 21a, parallel to strip 21, forming a similar choke, in order to maintain the symmetry of the device.

What is claimed:

1. A hybrid tee for coaxial lines comprising two coaxial collinear transmission lines having an inner conductor continuous between them and the ends of the outer conductor separated at the meeting point of the lines to provide open ends, a third coaxial line having the inner conductor connected to said continuous conductor and the outer conductor to both said open ends and a fourth coaxial line having its two conductors connected respectively by a chocking element to said open ends of said interrupted conductor of said collinear lines.

2. A hybrid tee for coaxial lines comprising two coaxial collinear transmission lines having an inner conductor continuous between them and the ends of the outer conductor separated at the meeting point of the lines to provide open ends, a third coaxial line having the inner conductor connected to said continuous conductor and the outer conductor to both said open ends and a fourth coaxial line having its two condutcors connected respectively by a choking element to said open ends of said interrupted conductor of said collinear lines and chokes connecting the outer conductors of said collinear lines with the outer conductors of said lateral lines.

3. A hybrid tee for coaxial lines comprising two coaxial collinear transmission lines having one conductor continuous between them and having the other conductors interrupted at the meeting point to provide open ends, a third coaxial line connected in shunt across both said first-mentioned transmission lines, and a fourth coaxial line having the inner conductor connected to the inner conductor of said first mentioned lines and its outer conductor connected to the outer conductor of said first mentioned lines, said last named connections constituting a parallel conductor line, and a choking means between each of said rst mentioned lines and said third and fourth lines.

References Cited in the tile of this patent UNITED STATES PATENTS 2,425,379 Lindenblad Aug. 12, 1947 2,454,907 Brown Nov. 30, 1948 2,507,915 Lindenblad May 16, 1950 2,532,736 Sheppard Dec. 10, 1950 2,547,054 Smullin Apr. 3, 1951 2,581,156 Weighton Jan. 1, 1952 2,594,167 Herold Apr. 22, 1952 2,690,540 Boothby Sept. 28, 1954 2,724,806 Tillotson Nov. 22, 1955 2,769,146 Alford Oct. 30, 1956 2,831,168 Smoll Apr. l5, 1958 FOREIGN PATENTS 224,079 Switzerland Feb. l, 1943 

